1. Field of the Invention
The present invention relates a blood flow velocity calculation apparatus and a method thereof, and more particularly, to an ophthalmic apparatus, an ophthalmic system, a processing apparatus, and a blood flow velocity calculation method used for an ophthalmic care.
2. Description of the Related Art
A scanning laser ophthalmoscope (SLO) which is an ophthalmic apparatus using the principle of a confocal laser microscope is a device that performs raster scan on an eye fundus with a laser as a measurement beam to obtain high resolution planar images at high speed from an intensity of return beam of the measurement beam.
Hereinafter, the device that captures such a planar image will be referred to as an SLO apparatus.
In recent years, the SLO apparatus has been able to acquire a planar image of a retina with an improved horizontal resolution by increasing a beam diameter of a measurement beam. However, with the increase in the beam diameter of the measurement beam, the acquired planar image of the retina has an issue of a low signal-to-noise (SN) ratio and a low resolution due to an aberration of a subject's eye.
To solve this issue, developed is an adaptive optics SLO apparatus including an adaptive optics system which measures the aberration of the subject's eye in real time using a wavefront sensor and corrects the aberration of the measurement beam or the return beam thereof occurring in the subject's eye using a wavefront correction device. In this way, the planar image having a high horizontal resolution can be acquired.
Moreover, a technique for successively acquiring the planar images of the retina with a high horizontal resolution using an adaptive optics SLO apparatus and calculating the blood flow velocity from a moving distance of a blood cell in a capillary vessel is discussed in “Joy A. Martin, Austin Roorda, Direct and Noninvasive Assessment of Parafoveal Capillary Leukocyte Velocity. Ophthalmology, 2005, 112:2219”. This document discusses a technique for calculating the blood flow velocity by utilizing a period (a period required for acquiring one planar image) from when the first planar image is acquired to when the second planar image is acquired.
As described above, the SLO apparatus is a device that performs raster scan on the retina with a measurement beam using a scanning unit to obtain planar images of the retina. Thus, an image capturing timing varies from position to position in the acquired planar image.
Therefore, the period (a period required for acquiring one planar image) from when the first planar image is acquired to when the second planar image is acquired is different from a period from when the scanning unit acquires a position (first position) of a blood cell in the first planar image to when the scanning unit acquires a position (second position) of the blood cell in the second planar image. Accordingly, the technique discussed in the above document cannot accurately calculate the blood flow velocity.